The incorporation of particles from millimeter-scale down to nanometers in size is ubiquitous in end-use products produced in industrial-scale quantities. A significant percentage of the particles used across all industries require that the surfaces be coated with a shell, layer, film, or other coating, ranging from sub-nanometer to hundreds of micrometers in thickness. For a variety of reasons, each sector or industry has determined that the incorporation of coated particles into the end-use product provides enough value, e.g., in the form of enhanced performance of the product, that the cost associated with each coating process is justified. Energy storage is one application where nanoscale coatings can significantly improve the uniformity and compatibility of surfaces, allowing for preferential transfer of beneficial ions or electrons across interfaces, while reducing the propensity for detrimental or corrosion promoting species from degrading or otherwise altering these interfaces.
The performance of passive electronic components such as capacitor technologies, including single-layer or multilayered ceramic capacitors (MLCC), electrolytic capacitors, polymer film capacitors, or emerging ultracapacitor and/or supercapacitor systems, relies on the quality of control across interfaces, which in turn defines the specification for capacitance, dielectric strength, breakdown voltage, dielectric loss, etc. Mechanisms to tailor and optimize all surfaces of all materials contained within the system leads to better control, definition, functionality or other specification of performance of any feature of each system.
Ceramic capacitors (bulk ceramic and MLCCs) have existed for quite some time, and the state of the art has progressively advanced to higher energy density, power density, lifetime/durability, and similar advances, all while occupying a decreasing footprint that trends with smaller sizes of integrated circuit technologies. Barium titanate (BaTiO3) is a commonly used dielectric material. Extensive work on this material has demonstrated that tailored bulk content (e.g., dopants, protonation, etc.) or utilization of surface coatings (e.g., Al2O3, SiO2, etc.) can be used to achieve higher breakdown voltages than untreated materials. Constantino et al. (U.S. Patent Application Publication No. 2001/0048969) discusses Al2O3-coated or SiO2-coated sub-micron BaTiO3 particles that are exemplary of these additional performance features. Many other tactics have been used to modify dielectric materials to achieve improved device properties.
Methods of producing compositionally-tailored ceramic/dielectric layers themselves using additive, layer-by-layer controlled techniques, even those as precise as Atomic Layer Epitaxy or Atomic Layer Deposition as described by Suntola et al. (U.S. Pat. No. 4,058,430), have been deployed to achieve similar effects (see, for example, Ahn, et al., U.S. Patent Application Publication No. 2011/0275163). In addition, techniques that cast or otherwise form a bulk layer consisting of a plurality of compositionally-tailored coated dielectric particles have also been described. See, for example, Constantino et al. in U.S. Patent Application Publication No. 2001/0048969. Coating processes for particles as precise as Atomic Layer Deposition is described by Lakomaa, et al. in the seminal demonstration of ALD coated particles: “Atomic layer growth of TiO2 on silica”, Applied Surface Science 60/61 (1992) 742-748.
Several years after the seminal publication of conformal metal oxide coatings on microfine powders (produced using sequential self-limiting gas phase reactions that occurred homogeneously on the surfaces of particles in a fixed bed of particles enclosed in a single batch reactor), additional patents have been issued pertaining to ALD and non-ALD techniques for producing high quality coatings on particles, including nanoparticles. As examples included herein by reference, Krause et al. (EP 0865819) discuss methods of encapsulating particles using fluidized beds; Cansell et al. (U.S. Pat. No. 6,592,938) discuss methods of coating particles using organometallic precursors that are individually known to undergo self-limiting reactions under traditional ALD conditions. Cansell further discusses (see U.S. Pat. No. 7,521,086) as to how the latter coating technique could similarly be utilized for the production of a metal oxide encapsulated BaTiO3.
As described by King et al. (US 20110236575), vapor deposition processes are usually operated batch-wise in reaction vessels such as fluidized bed reactors, rotary reactors and V-blenders, amongst others. Batch processes have significant inefficiencies when operated at large scale. One of the disadvantages of batch processes is that the reactor throughput is a function of the total particle mass or volume loaded into a certain sized vessel for a given process, the total process time (up-time), and the total time between processes (down-time) to load, unload, clean, prepare, etc. In addition, batch processes incur large down-times because at the end of each batch the finished product must be removed from the reaction equipment and fresh starting materials must be charged to the equipment before the subsequent batch can be produced. Equipment failures and maintenance add to this downtime in batch processes.
Moreover, relatively speaking, batch process equipment tends to be very large and expensive. The need to operate these processes under vacuum adds greatly to equipment costs, especially as equipment size increases. Because of this, equipment costs for batch processes tend to increase faster than operating capacity.
Another problem that occurs as the process equipment becomes larger is that it becomes more difficult to maintain uniform reaction conditions throughout the vessel. For example, temperatures can vary considerably within a large reaction vessel. It is also difficult to adequately fluidize a large mass of particles, specifically nanoparticles. Issues such as these can lead to inconsistencies and defects in the coated product.
In vapor deposition processes such as ALD and Molecular Layer Deposition (MLD), the particles are contacted with two or more different reactants in a sequential manner. This represents yet another problem for a batch operation. For a traditional batch process, all cycles are performed sequentially in a single reaction vessel. The batch particle ALD process incurs additional down-time due to more frequent periodic cleaning requirements, and the reaction vessels cannot be used for multiple film types when cross-contamination could be problematic. In addition, the two sequential self-limiting reactions may occur at different temperatures, requiring heating or cooling of the reactor between cycle steps in order to accommodate each step.
The throughput for a batch process can be increased either by building larger reaction vessels and/or operating identical reaction vessels in parallel. The capital cost to counteract this down-time from a throughput perspective is to build a larger reaction vessel. With larger vessels, localized process conditions, including internal bed heating, pressure gradients, mechanical agitation to break up nanoparticle aggregates, and diffusion limitations amongst others, become more difficult to control.
Furthermore, there is a practical maximum reaction vessel size when performing ALD processes on fine and ultra-fine particles, which limits the throughput for a single batch reactor operating continually. In general, the time duration for the process of producing a given amount of coated materials equals the up-time plus down-time. There is also a practical maximum allowable in capital expense to fabricate an ALD-coated particle production facility, which effectively limits the number of batch reactors that can operate identical processes in parallel. With these and other constraints, there are practical throughput limitations that prohibit the integration of some particle ALD processes at the industrial scale.
King et al. (U.S. Patent Application Publication No. 2011/0236575) discusses a high-rate “Spatial ALD” manufacturing process and apparatus for coating particles in semi-continuous fashion using an array of isolated vessels with counter-current gas-solids transport. As described in King et al., one example where a semi-continuous coated particle manufacturing process is desirable is a facility that utilizes particle ALD to produce fine or ultra-fine passivated titanium dioxide particles used as pigments in paints, plastics, paper, etc. Another example is a facility that utilizes particle ALD to produce coated fine or ultra-fine particles for cathodes, anodes, dielectrics, metals, polymers, semiconductors and other ceramics for integration into power systems devices including, but not limited to, batteries, capacitors, varistors, thyristors, inverters, transistors, light emitting diodes and phosphors, photovoltaic, and thermoelectric devices. Particle ALD produced powders for the pigment and power systems industries can significantly improve the performance of the end-use products, which can be cost competitive if produced at high annual throughputs.
Van Ommen et al. (U.S. Patent Application Publication No. 20120009343) discusses another high-rate “Spatial ALD” process and apparatus to coat particles in a fully continuous co-current gas-solids transport scheme. Each of these methods has its own ascribed operating cost. These methods are suitable for the manufacture of particular coated particles. In addition, each of these methods is believed to be superior and economically more viable to traditional batch (or “temporal”) ALD coating methods. Fotou et al. (Sequential Gas-Phase Formation of Al2O3 and SiO2 Layers on Aerosol-Made TiO2 Particles” Advanced Materials (1997), 9, No. 5, 420-423) discuss methods of producing nanocoatings on submicron particles by exposing reactive precursors to the surfaces of particles using continuous-flow Chemical Vapor Deposition techniques. However, the consistency, uniformity and thickness do not lend themselves easily to less than 5 nm coatings on submicron-sized particles.
It is expected that a thin film coating (e.g., 5 nm or less) on nanoparticles that are used in passive electronic components will provide a significant protection from degradation and/or oxidation of nanoparticles while maintaining substantially all of its electronic function.
Accordingly, there is a need for thin film coated nanoparticles and methods for producing and using the same in passive electronic components.